The present invention relates to an optical pickup apparatus by which the recording and/or reproducing of the information can be compatibly conducted on different kinds of optical information recording media, and to an objective optical element.
Recently, in the optical pickup apparatus, the wavelength-shortening of the laser light source used as the light source for the reproducing of the information recorded in the optical disk, or the recording of the information in the optical disk is advanced, for example, a laser light source of the wavelength 400-420 μm, such as the blue violet semiconductor laser, or the blue violet SHG laser to conduct the wavelength conversion of the infrared semiconductor laser by using the second harmonic wave, is putting to a practical use. In the case where these blue violet laser light sources are used, when the objective lens of the same numerical aperture (NA) as DVD (Digital Versatile Disc) is used, the recording of the information of 15-20 GB becomes possible for the optical disk of 12 cm diameter, and when NA of the objective lens is increased to 0.85, the recording of the information of 23-25 GB becomes possible for the optical disk of 12 cm diameter. In the present specification, the optical disk and photo-magnetic disk using the blue violet laser light source are generally called as “high-density optical disk”, hereinafter.
Hereupon, in the high-density optical disk for which the objective lens of NA 0.85 is used, because the coma generated due to the skew of the optical disk is increased, there is the lens whose protective layer is designed thinner than the case in DVD (0.1 mm to 0.6 mm of DVD), and in which the coma due to the skew is reduced. Hereupon, not only saying that the recording and reproducing of the information can be adequately conducted on such type of the high-density optical disk, it can not be said that a value as a product of the optical disk player/recorder is enough. In the present time, based on a actuality that DVD Or CD (Compact Disk) in which various information are recorded, is put in a market, it is not enough only that the recording/reproducing of the information can be conducted on the high-density optical disk, but, a fact that, for example, also for DVD or CD which is owned by the user, the recording/reproducing of the information can be adequately conducted in the same manner, can introduce to an actuality that a value of the product as the optical disk player/recorder for the high-density optical disk is enhanced. From such a background, it is requested that, while the optical pickup apparatus mounted in the optical disk player/recorder for the high-density optical disk keeps the compatibility also for any one of the high-density optical disk and DVD, further, CD, the apparatus has the performance which can adequately record/reproduce the information.
As a method by which, while the apparatus has the compatibility also for any one of the high-density optical disk and DVD, further, CD, the information can be adequately recorded/reproduced, a method by which the optical system for the high-density optical disk and the optical system for DVD or CD are selectively switched corresponding to the recording density of the optical disk for which the information is recorded/reproduced, is considered, however, because a plurality of optical systems are necessary, it is disadvantageous for the size reduction, and further, the cost is increased.
Accordingly, in order to simplify the structure of the optical pickup apparatus and to intend the cost reduction, also in the optical pickup apparatus having the compatibility, it is preferable that the optical system for the high-density optical disk and the optical system for DVD or CD are made in common and the number of optical parts constituting the optical pickup apparatus is reduced at most. Then, it is most advantageous for the simplification of the structure of the optical pickup apparatus, and for the cost reduction that the objective optical system arranged in opposite to the optical disk is made in common. Hereupon, in order to obtain the objective optical system common for a plurality of kinds of optical disks whose recording/reproducing wavelengths are different from each other, it is necessary that the phase structure having the wavelength dependency of the spherical aberration is formed in the objective optical system.
In the Patent Document 1, an objective optical system which has the diffractive structure as the phase structure, and can be commonly used for the high-density optical disk and the conventional DVD and CD, and an optical pickup apparatus in which this objective optical system is mounted are written.
[Patent Document 1] Europe Unexamined Patent No. 1304689
However, because the objective optical element used for the optical pickup apparatus by which the recording and/or reproducing of the information is compatibly conducted on 3 different optical disks written in the above Patent Document 1 is the structure in which the objective lens and the optical element in which the phase structure is formed are combined in the optical axis direction, it is comparatively large sized and weighted, and the burden of an actuator for driving it at the time of the tracking or focusing, is large, accordingly, there is a problem that it is necessary that a large sized actuator is used, and the size reduction or energy saving of the optical pickup apparatus is not intended.